The introduction of High Speed Packet Access (HSPA) greatly improves the achievable bit rate over the air interface, but it presents challenges to be solved in the Wideband Code Division Multiple Access (WCDMA) radio access network (RAN), which is a transport network. FIG. 1 illustrates a WCDMA network 10 with one or more user equipments (UEs) 12 communicating with a WCDMA radio access network (WCDMA RAN) over a Uu air interface, and one or more core networks 18 communicate with radio network controllers (RNCs) 16 in the WCDMA RAN over an Iu interface. WCDMA RAN is also called Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (UTRAN). The WCDMA RAN handles all tasks that relate to radio access control such as radio resource management and handover control. The core network connects the access network to one or more external networks (PSTN, Internet, etc.). The user equipment (sometimes called a Mobile Terminal or Mobile Station) 12 is connected to one or more radio base stations (Node Bs) 13 over the WCDMA air interface (Uu). One or more base stations are coupled to an RNC 16 over an Iub interface, and the RNCs communicate over an Iur interface. During soft handover, one UE can communicate with several Node Bs simultaneously.
According to the WCDMA RAN specifications by the 3rd Generation. Partnership Project (3GPP), all radio network functions and protocols are separated from the functions and protocols in the transport network layer (TNL). The WCDMA RAN Transport Network transmits data and control information between the Radio Network Controller (RNC) and the Node Bs (Iub interface) or between RNCs (Iur interface). For an active UE connection, there is a single Serving RNC (SRNC) that terminates the user and control plane protocols of that UE. The transport network layer provides data and signaling bearers for the radio network application protocols between RAN nodes and includes transport network control-plane functions for establishing and releasing such bearers when instructed to do so by the radio network layer.
The first release of the 3GPP specification was called Release 99. FIG. 2 shows the Release 99 protocol stack at the Iub interface for transferring data streams on common transport channels (CCH) and dedicated transport channels (DCH) to the air interface. The acknowledged retransmission mechanism of the radio link control (RLC) protocol ensures reliable transmission of loss-sensitive traffic over the air interface. RLC provides three types of service to the upper layers: transparent (no additional protocol information), unacknowledged (delivery not guaranteed), and acknowledged data transfer mode. The normal RLC mode for packet-type services is the acknowledged mode, so RLC works in this mode in case of HSPA as well. The RLC protocol is used by both signaling radio bearers and radio bearers for packet-switched data services, but not by radio bearers for circuit-switched services. RLC Acknowledged Mode (AM), which is a Selective Repeat Automatic Repeat-request (SR-ARQ) protocol, provides transport service to upper layers between UE and the RNC. RLC AM does not include congestion control functionality because it assumes that RLC Protocol Data Units (PDUs) are transmitted by the Medium Access Control—dedicated (MAC-d) layer according to the available capacity allocated on the air interface Uu and the Transport Network. RLC status messages are sent regularly based on preset events and trigger retransmission of all missing PDUs. The receiver side detects missing RLC PDUs based on gaps in the PDU sequence numbers. The receiver side RLC requests retransmission by sending back a status PDU, informing which PDUs within the receiving window have been acknowledged (ACK) or lost (negative ACK, NACK). Upon reception of a status message, the sender can slide its transmission window (Tx window) if one or more in-sequence frames are acknowledged, so that new PDUs can be sent. If there are NACKs in the status message, the sender retransmits the missing PDUs giving them priority over new ones. Several unsuccessful retransmissions of the same PDU trigger an RLC reset, and the whole RLC Tx window is discarded.
FIG. 2 shows a user plane protocol stack between the RNC and Node B for Release 99. The MAC-d protocol forms sets of transport blocks in the air interface and schedules them according to the timing requirements of WCDMA. Each scheduled period, called a transmission time interval (TTI), is 10 ms in length or multiples thereof. Release 99 WCDMA radio connections, or radio access bearers (RAB), have practical bit rate values between 8 and 384 kbps. The size of the MAC transport blocks and length of the TTI are RAB-specific. For data transfer over the Iub interface, the MAC transport blocks are encapsulated into Iub frames according to the Iub user-plane (UP) protocol for CCH or DCH data streams. Each Iub user-plane data stream needs a separate transport network connection between the RNC and Node B. The transport network thus establishes one transport network layer connection for each data stream. In FIG. 2, an optional TNL switch may used for building/aggregating transport networks.
In response to the increased need for higher bit rate and more efficient transmission of packet data over cellular networks, the WCDMA 3GPP Release 5 extended the WCDMA specification with the High Speed Downlink Packet Access (HSDPA), and Release 6 introduced Enhanced Dedicated Channel (E-DCH), often referred as Enhanced Uplink (EUL) or High Speed Uplink Packet Access (HSUPA). HSDPA and HSDPA together are called HSPA. In HSPA, certain parts of radio resource control are moved from the RNC to the Node Bs. In 3GPP Release 7, higher-order modulation and multiple input multiple output (MIMO) are introduced for HSDPA to further improve the achievable bit rate.
Although transport network bandwidth can be reserved using admission control for Dedicated Channels (DCHs) traffic in the access transport network, individual data packet “flow” bandwidth reservation is not efficient for HSPA because air interface throughput is much higher and fluctuates more. The term “flow” corresponds to a logical connection in the radio access transport network that carries data packets associated with a UE connection. Packets associated with a flow are carried using one or more communications channels on a physical link connecting nodes in the transport network. Because bandwidth reservation is not used, congestion may occur in both the Iub transport network and over the air interface. Congestion occurs when the available communications capacity is exceeded by the sum of the current data packet flow rates.
In the current WCDMA architecture, the TCP protocol layer in the UE and in the uplink server cannot efficiently resolve a congestion situation in the radio access network because RLC AM retransmissions between the RNC and the UE hide congestion situations from TCP. Congestion is particularly a problem given the increased air interface capacity provided by HSPA. That increased air interface capacity does not necessarily mean there is increased Iub transport network capacity. Network operators often upgrade the Node Bs first and delay the upgrade of the transport network until there is significant HSPA traffic. In some cases, the cost of Iub transport links is still high, and thus, it is a common scenario that throughput is limited by the capacity available on the Iub transport network links and not by the capacity of the air interface. For example, a typical scenario is that the Node B is connected to the RNC through an E1 link of capacity about 2 Mbps, and the available Uu capacity for HSDPA can be significantly larger that this 2 Mbps. So a single UE given good radio conditions can overload the radio transport network (TN). “Bottleneck” transport network links or nodes can therefore be a serious problem.
TCP “slow start” normally quickly increases a TCP congestion window size to its maximum, and it is kept at that value. Due to RLC retransmissions, TCP does not experience packet loss. In case of transport network congestion, the RLC layer keeps retransmitting lost PDUs until they are successfully received (or until RLC resets). Unfortunately, these retransmissions increase the congestion level, and TCP is only notified about the transport network congestion at a significantly later time. Accordingly, because the RLC layer does not have congestion control functionality and TCP congestion control does not operate efficiently in the transport network, HSPA data packet flow control is necessary. Although fair sharing of radio resources over the Uu air interface is the task of the Uu scheduler in the Node B, the Uu scheduler can not solve the bottleneck problem in the transport network.
To deal with this transport network bottleneck problem, a flow control (FC) mechanism may be introduced in the RNC and/or Node B to regulate each data packet flow. An important goal for transport network flow congestion detection and control is to fairly allocate the transport network communication resources (e.g., bit rate) to ongoing data packet flows based on the available link capacity in the transport network. But the fairness of HSPA flow control depends on the fairness of transport network congestion detection. For example, an Additive Increase Multiplicative Decrease (AIMD) type congestion control procedure requires synchronous congestion signals for fair bandwidth sharing. Synchronous congestion signals means that when a transport network link is congested, all the flows using that link need to detect or otherwise be aware of the transport network congestion situation. But if a first group of data packet flows detects transport network congestion on a particular link and a second group of flows using that same link do not, then the bit rate of the two groups of will differ significantly. The “lucky” flows that do not detect congestion will have higher bit rates and a larger bandwidth share than the unlucky flows. This bandwidth disparity between flows is caused by unfair or inadequate transport network congestion detection.